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          <h1 class="post-title" itemprop="name headline">ObjC之Block解读</h1>
        

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        <h3 id="1-类型"><a href="#1-类型" class="headerlink" title="1. 类型"></a>1. 类型</h3><p>根据block在内存中的位置，分为如下三种</p>
<ul>
<li>NSGlobalBlock<ul>
<li>位于text段</li>
<li>不引用外部成员变量或局部变量（引用全局或静态也是NSGlobalBlock）<pre>
void (^block)(int a, int b) = ^(int a, int b){
//此处除了形参，不能使用外部变量
int c = a+b;
};</pre></li>
</ul>
</li>
<li>NSStackBlock<ul>
<li>位于栈内存</li>
<li>在函数中直接通过^{}定义的</li>
<li>不管block是否使用外部变量，都是NSStackBlock<pre>
NSLog(@"block = %@", ^{
  //此处不管是否访问外部变量，都是NSStackBlock
})</pre></li>
</ul>
</li>
<li>NSMallocBlock<ul>
<li>位于堆内存</li>
<li>引用外部变量</li>
<li>ARC模式下会将block从栈拷贝到堆上，而MRC中需要使用copy<pre>
int c;
void (^block)(int a, int b) = ^(int a, int b){
//此处引用了外部变量c
int d = c+a+b;
};</pre>

</li>
</ul>
</li>
</ul>
<h3 id="2-引用外部变量-针对NSStackBlock、NSMallocBlock"><a href="#2-引用外部变量-针对NSStackBlock、NSMallocBlock" class="headerlink" title="2 引用外部变量(针对NSStackBlock、NSMallocBlock)"></a>2 引用外部变量(针对NSStackBlock、NSMallocBlock)</h3><h4 id="2-1-引用外部基础类型变量"><a href="#2-1-引用外部基础类型变量" class="headerlink" title="2.1 引用外部基础类型变量"></a>2.1 引用外部基础类型变量</h4><pre>
@interface TestViewController () {
    int _count; //成员变量
}
@end

...

int g_count = 100; //全局变量

- (void)test {
    ....

    static int s_count = 100; //静态变量
    int count = 101; //局部变量
    NSLog(@"g_count= %p", &g_count);
    NSLog(@"s_count= %p", &s_count);
    NSLog(@"_count = %p", &_count);
    NSLog(@"count= %p", &count);
    ^{
        NSLog(@"block g_count= %p", &g_count);
        NSLog(@"block s_count= %p", &s_count);
        NSLog(@"block _count = %p", &_count);
        NSLog(@"block count= %p", &count);
    }();
    ...
}
</pre>
输出:
<pre>
2015-10-09 23:09:55.564 BlockTest[64349:778572] g_count= 0x1077d1390
2015-10-09 23:09:55.565 BlockTest[64349:778572] s_count= 0x1077d1394
2015-10-09 23:09:55.565 BlockTest[64349:778572] _count = 0x7fe240ea4478
2015-10-09 23:09:55.565 BlockTest[64349:778572] count= 0x7fff5842ed5c
2015-10-09 23:09:55.565 BlockTest[64349:778572] block g_count= 0x1077d1390
2015-10-09 23:09:55.565 BlockTest[64349:778572] block s_count= 0x1077d1394
2015-10-09 23:09:55.566 BlockTest[64349:778572] block _count = 0x7fe240ea4478
2015-10-09 23:09:55.566 BlockTest[64349:778572] block count= 0x7fff5842ed58
</pre>

<ul>
<li>对于局部变量，block将会对其进行copy</li>
<li>static变量、全局变量，block不对其进行copy</li>
</ul>
<h4 id="2-2-外部对象类型变量"><a href="#2-2-外部对象类型变量" class="headerlink" title="2.2 外部对象类型变量"></a>2.2 外部对象类型变量</h4><h5 id="MRC模式下"><a href="#MRC模式下" class="headerlink" title="MRC模式下"></a>MRC模式下</h5><ul>
<li>全局对象、静态对象，在block copy，不会进行retain</li>
<li>成员变量，在block copy 不会对成员变量进行retain，<code>但是会对self进行retain</code></li>
<li>局部变量，在block copy 会进行retain</li>
</ul>
<h5 id="ARC模式下"><a href="#ARC模式下" class="headerlink" title="ARC模式下"></a>ARC模式下</h5><ul>
<li>局部变量，block会copy其指针，并强引用一次</li>
<li>静态变量、全局变量会强引用一次</li>
<li>当block被释放，则相应的引用减1</li>
<li>成员变量，在block copy 不会对成员变量进行retain，<code>但是会对self进行retain</code></li>
</ul>
<h3 id="3-注意事项"><a href="#3-注意事项" class="headerlink" title="3. 注意事项"></a>3. 注意事项</h3><ul>
<li>与其他objc对象不同，block默认是在栈上分配的，因此需要注意copy  <pre>
  “As an optimization, block storage starts out on the stack—just like blocks     themselves do.”</pre></li>
<li>NSSet和NSArray 初始化(arrayWithObjects, initWithObjects)只对第一个block进行copy(验证了下字典对象是ok的)  <pre>
  [NSMutableArray arrayWithObjects:
          ^{ NSLog(@"this is block 1:%i", num); }, //NSMallocBlock
          ^{ NSLog(@"this is block 1:%i", num); }, //NSStackBlock
          nil];</pre></li>
<li><p>循环引用</p>
   <pre>
  //如下代码将会导致循环引用
  self.block = ^{
      //此处对成员变量_testObject1进行了引用，会导致对self进行了retain(参考2.2)
      NSLog(@"block ==========%@", _testObject1);
  };</pre>

<ul>
<li>此处解决方法有<ul>
<li>(1) self.block = nil</li>
<li>(2) 使用weak或者__unsafe_unretained<pre>
__weak typeof(self) weakSelf = self;
//__unsafe_unretained typeof(self) weakSelf = self;
self.block = ^{
  NSLog(@"block ==========%@", weakSelf.testObject1);
};</pre>

</li>
</ul>
</li>
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              <div class="post-toc-content"><ol class="nav"><li class="nav-item nav-level-3"><a class="nav-link" href="#1-类型"><span class="nav-number">1.</span> <span class="nav-text">1. 类型</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#2-引用外部变量-针对NSStackBlock、NSMallocBlock"><span class="nav-number">2.</span> <span class="nav-text">2 引用外部变量(针对NSStackBlock、NSMallocBlock)</span></a><ol class="nav-child"><li class="nav-item nav-level-4"><a class="nav-link" href="#2-1-引用外部基础类型变量"><span class="nav-number">2.1.</span> <span class="nav-text">2.1 引用外部基础类型变量</span></a></li><li class="nav-item nav-level-4"><a class="nav-link" href="#2-2-外部对象类型变量"><span class="nav-number">2.2.</span> <span class="nav-text">2.2 外部对象类型变量</span></a><ol class="nav-child"><li class="nav-item nav-level-5"><a class="nav-link" href="#MRC模式下"><span class="nav-number">2.2.1.</span> <span class="nav-text">MRC模式下</span></a></li><li class="nav-item nav-level-5"><a class="nav-link" href="#ARC模式下"><span class="nav-number">2.2.2.</span> <span class="nav-text">ARC模式下</span></a></li></ol></li></ol></li><li class="nav-item nav-level-3"><a class="nav-link" href="#3-注意事项"><span class="nav-number">3.</span> <span class="nav-text">3. 注意事项</span></a></li></ol></div>
            

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